1. Field of the Invention
The present invention relates to an apparatus for determining a removed amount of a wafer, and more particularly to an apparatus for determining a removed amount of a wafer during processing in a grinding/polishing apparatus which grinds or polishes a face of the wafer by moving a wafer holding table which holds the wafer thereon and a grinding/polishing member relatively to one another while pressing the face of the wafer against the grinding/polishing member.
2. Description of Related Art
To determine thickness variations (a removed amount) of a wafer during polishing of the wafer, in a polishing apparatus for polishing a surface of a wafer such as a semiconductor wafer, is important for detecting the end point of the polishing of the wafer, improving the quality of the polished wafer, and performing the closedloop control of the apparatus.
In the conventional polishing apparatus such as a chemical mechanical polishing (CMP) apparatus, since the direct measurement for the thickness variations of the wafer is difficult, the removed amount of the wafer is estimated by means of electric current variations of a motor which drives a turn table, on which a polishing pad is provided, or a wafer fixing spindle (the motor current measuring method). In this method, however, there is a disadvantage in that accuracy of estimation is poor because the estimation is very indirect.
In a polishing apparatus which polishes a wafer while moving a wafer holding table, which holds the wafer thereon, toward the polishing pad, the removed amount of the wafer can be estimated by means of positional variations of the wafer holding table. In this method, it is required that the position of a polishing face of the polishing pad is constant during polishing; however, the position of the polishing face varies as a result of a warp of the turn table and wear of the polishing pad. Then, the removed amount of the wafer can not be determined accurately by means of the positional variations of the wafer holding table only.